The primary problem to overcome in utilizing a closed mold system for casting of objects such as contact lenses where surface appearance, edge regularity and surface finish are important is the compensation for the inevitable shrinkage that occurs on polymerization. With most monomeric materials the volumetric shrinkage on polymerization is in the range of 12 to 22%. This shrinkage has heretofore prevented satisfactory casting of objects such as contact lenses from vinyl monomers such as acrylate and methacrylate esters, vinyl pyrollidone, substituted or unsubstituted acrylamides or methacrylamides and the like. For example, as taught in U.S. Pat. No. 3,660,545 (col 1 & 2) a polymerizing acrylate, or methacrylate ester mixture held in a closed glass mold invariably would pull away from at least one mold surface and cause the formation of surface voids which rendered the cast object unsuitable as a lens.
In a further prior art method, (see U.S. Pat. No. 3,660,545) the polymerizing mixture was held between concave and convex glass mold members having an annular gap interposed between them which decreased as polymerization occurred, but again an irregular edge configuration resulted from polymerization shrinkage. This edge portion could be removed by cutting but this resulted in an edge of appreciable thickness which tended to catch the eyelid during movement of the latter, and caused displacement of the lens.
These difficulties with shrinkage compensation therefore have heretofore precluded lens production by casting in closed mold systems and other methods of producing contact lenses of crosslinked polymeric materials such as spin casting and machining and polishing have been employed.
In the spin casting process as taught in U.S. Pat. No. 3,408,429 and U.S. Pat. No. 3,496,254 the polymerizing mixture is contained in a rotating open mold having a concave surface. The anterior, convex surface of the lens is thus formed by the mold surface, and the posterior, concave surface of the lens is formed as a result of centrifugal forces, surface tension of the polymerizing mixture and other factors such as the size and shape of the mold, the volume of the polymerizing mixture, the condition of the mold surface, etc. The concave surface of the lens thus formed is approximately parabolic in shape and many factors must be carefully controlled to produce reproducible shapes. Lenses produced by spin casting usually require post-polymerization edge finishing and optical quality is not of the highest since there is no truly spherical optical zone due to the aspheric nature of the posterior surface.
Most manufacturers of contact lenses of crosslinked polymeric materials use a traditional machining or mechanically working and polishing of lens blanks as taught in U.S. Pat. No. 3,361,858. This method has the advantage of being able to "tailor make" lens curvatures and powers to any desired degree and to furnish high optical quality. The method suffers from the disadvantage of requiring highly skilled artisans for quality production.
It is the object of this invention to provide a method for producing cast lenses in a closed mold system, such lenses having the desirable optical and fitting characteristics of machined and polished lenses.